Organisms respond to changes in their internal and external environments (AQA A2 Biology) PART 3 of 9 TOPICS
Control of heart rate:
Action potentials originate in the sinoatrial node and travel across the wall of the atrium to the atrioventricular node on the right side of the heart. This passes slowly giving time for the atria to contract and empty all the blood into the ventricle. This is then passed along the atrioventricular bundle into the interventricular septum along the bundle of His. This bundle separates into two at the apex (bottom) of the ventricles and goes upwards along each of the ventricle walls. The action potentials travel along this pathway and is carried deeper into the ventricular walls by the Purkinje fibres. This provides unison of a strong contraction by both the ventricles and allowing the ventricles to empty properly.
Chemoreceptors, located in the aorta and the carotid artery, monitor the CO2 levels, O2 levels and pH levels in the blood. When the receptor picks up the stimulant it transfers it to the sensory neurone to the medulla oblongata (also known as medulla) which also has chemoreceptors. The impulse then travels along the sympathetic neurone or parasympathetic neurone depending on the stimuli to the sinoatrial node. The effector, the cardiac muscle, can have two responses depending on the stimulus and route it takes:
- If there are low levels of O2 and pH with high levels of CO2 then the impulse will go through the sensory neurone to the medulla and through the sympathetic neurone releasing noradrenalin from the sinoatrial node (an excitatory neurotransmitter) which makes the heart rate faster.
- If there are high levels of O2 and pH with low levels of CO2 then the impulse will go through the sensory neurone to the medulla and through the parasympathetic neurone releasing acetylcholine from the sinoatrial node (an inhibitory neurotransmitter) which makes the heart rate slower. This is so the levels of O2 and pH can lower and CO2 levels can be raised.
Baroreceptors, located in the carotid artery and aorta only, detect changes in the pressure in the blood. The routes are exactly the same for which the nerve impulses take:
- If there is high blood pressure the route it follows is through the parasympathetic neurone which releases acetylcholine from the sinoatrial node slowing the heart rate down to lower the blood pressure.
- If there is low blood pressure the route it follows is through the sympathetic neurone which releases noradrenalin from the sinoatrial node making the heart rate faster to raise the blood pressure.